In an active matrix type organic electroluminescent (EL) display apparatus, each pixel is formed including a pixel circuit generally having, in addition to an organic EL element, two transistors and one capacitor (2T1C) serving as elements for driving the organic EL element. More specifically, a driving TFT which drives the organic EL light emitting element, a writing TFT which controls a data voltage to be applied to the driving TFT, and a storage capacitor which stores the data voltage are provided.
A channel of a TFT is generally formed of a thin film semiconductor such as amorphous silicon, microcrystal silicon, poly-crystalline silicon, an oxide semiconductor, an organic semiconductor, and so on.
In this case, a TFT drain current Id is determined by the following formula:Id=0.5*(μCch*(W/L))*(Vgs−Vth)2 Here, μ represents a carrier mobility, Cch represents a channel capacitance, W and L represent a channel width and a channel length, respectively, Vgs represents a gate-source bias, and Vth represents a threshold voltage.
Here, degradation with time associated with a variation in mobility and a threshold voltage and application of bias is observed in any semiconductors. Also, drain current of the driving TFT to be supplied to the light emitting element depends on the mobility and the threshold voltage of the driving TFT. Accordingly, a variation in the mobility and the threshold voltage of a driving TFT in each pixel results in a variation of light emission brightness of each pixel with respect to a certain target brightness signal voltage input, which leads to non-uniform display characteristics.
In order to address the above problem, attempts to compensate for mobility and a threshold value of a driving TFT to thereby obtain uniform transconductance have been proposed. Such attempts include a Vth compensation circuit for correcting the threshold voltage of a driving TFT (U.S. 2007-285359), current writing drive for correcting a threshold voltage and mobility (U.S. Pat. No. 6,229,506), and so on.
In the example described in U.S. 2007-285359, a threshold voltage of a driving TFT, which has been previously detected, is superposed on a data voltage and the resulting voltage is applied between gate and source of the driving TFT, to thereby cancel effects of the threshold voltage on the drain current of the driving TFT, so that driving current which does not depend on Vth is supplied to a light emitting element. In this case, while a variation of mobility is not compensated, sufficient display uniformity can be achieved when effects of a variation of mobility upon the drain current are small.
In the example described in U.S. Pat. No. 6,229,509, a target brightness current is input to drain of a driving TFT in a state where the drain and gate of the driving TFT are short-circuited, to thereby induce a gate voltage required for applying a target current to the gate of the driving TFT. In this example, as not only a threshold voltage but also a variation of mobility are corrected, excellent display uniformity can be obtained even when a variation of mobility.
The two conventional examples described above are proposed attempts aimed at uniformity of drain current of a driving TFT, which is supplied to the light emitting element. In the actual display apparatuses, however, in addition to uniformity of the driving current to be supplied to the light emitting element, uniformity of current light emission efficiency of the light emitting element imposes significant effects on uniformity of the display brightness.
Normally, in driven-by-current type light emitting elements such as organic EL, a phenomenon in which the light emission efficiency is lowered in accordance with light emission of the elements can be observed. Recently, with the improvement of organic EL materials and light emitting element structures, organic EL elements having a constant current light emission brightness-half-life of tens of thousands of hours or more under average use conditions of a display apparatus are being reported.
In the applications of display apparatuses in which averaged use is expected for a whole display region, as the brightness is reduced substantially uniformly over the whole display screen, the brightness-half-life can be considered as an apparatus life. In this case, with the brightness-half-life of several tens of thousands of hours or more, no significant problems would occur in general applications.
However, in the applications of display apparatuses in which use of a large number of simple geometric patterns is assumed, such as mobile terminals, game terminals, PC monitor applications, and so on, the whole screen is used at random and uniform degradation cannot be expected.
In these applications of display apparatuses, a specific region in the screen and a region adjacent thereto are used with different frequencies and different brightness over a long period of time, which can result in a reduction in the light emission efficiencies which vary among different regions. This can cause image persistence of patterns on the screen, which is recognized by a viewer more sensitively than when the brightness of the whole screen is reduced uniformly. In most severe cases, a border between adjacent regions can be recognized if the difference of the brightness is approximately 2 or 3%. It is considered that such image persistence can be recognized with the brightness difference of approximately 5%, although it depends on the application of display apparatuses and patterns of image persistence.
As such, even if current supplied from the driving TFT is corrected in some manner, uniform brightness of the display apparatus can be inhibited due to a significant variation of light emission efficiency of the light emitting elements. In particular, in the applications of display apparatuses in which the product life depends on an image persistence life, it is necessary to correct a variation of light emission efficiency of the light emitting elements so as to secure a sufficiently long product life.
In order to correct degradation of a light emitting element itself, it is necessary to measure the light emission efficiency. Fish et al “Optical Feedback for AMOLED Display Compensation using LTPS and a-Si:H Technologies” SID 05 Digest, pgs 1340-1343 and Shin et al “A New Stable a-Si:H TFT Pixel for AMOLED by Employing the a-Si:H TFT Photo Sensor”, SID 08 Digest, pgs. 1211-1214 propose to correct a reduction of the light emission efficiency (optical compensation) by providing a photodetector in a pixel and controlling a light emission period in accordance with the light emission intensity of an organic EL element. A key to this method is requirements for a photodetector. Specifically, it is required that a photodetector should have a sufficient sensitivity, exhibit good linearity with respect to input light, and have stable and uniform characteristics. While use of an off-biased amorphous silicon TFT or PIN diode has been proposed as a photodetector, there are problems that, for the former, the linearity of sensitivity and light current need to be improved and that, for the latter, an additional process need to be added to the manufacturing process. Further, due to the effects of non-linearity and parasitic capacitance of the proposed pixel circuit, it is difficult to realize completely uniform brightness characteristics. For example, Shin et al “A New Stable a-Si:H TFT Pixel for AMOLED by Employing the a-Si:H TFT Photo Sensor”, SID 08 Digest, pgs. 1211-1214 discloses that a reduction in brightness caused by degradation of the light emission efficiency when optical compensation is performed can be reduced to ⅓ compared to when no optical compensation is performed.